Phosphodiesterases (PDE) play an important role in various biological processes by hydrolysing the key second messengers adenosine and guanosine 3′,5′-cyclic monophosphates (cAMP and cGMP respectively) into their corresponding 5′-monophosphate nucleotides. Therefore, inhibition of PDE activity produces an increase of cAMP and cGMP intracellular levels that activate specific protein phosphorylation pathways involved in a variety of functional responses. At least eleven isoenzymes of mammalian cyclic nucleotide phosphodiesterases, numbered PDE 1 through PDE 11, have been identified on the basis of primary structure, substrate specificity or sensitivity to cofactors or inhibitory drugs.
Among these phosphodiesterases, PDE7 is a cAMP-specific PDE. The biochemical and pharmacological characterization showed a high-affinity cAMP-specific PDE (Km=0.2 μM), that was not affected by cGMP potent selective PDE isoenzyme inhibitors.
PDE7 activity or protein has been detected in T-cell lines, B-cell lines, airway epithelial (AE) cell lines and several foetal tissues.
Increasing cAMP levels by selective PDE7 inhibition appears to be a potentially promising approach to specifically block or modulate T-cell and B-cell mediated immune responses. Further studies have demonstrated that elevation of intracellular cAMP levels can modulate inflammatory and immunological processes. This selective approach could presumably be devoid of the side effects associated with known selective PDE inhibitors (e.g. PDE3 or PDE4 selective inhibitors) and which limit their use.
A functional role of PDE7 in T-cell activation has also been disclosed; therefore selective PDE7 inhibitors are candidates for the treatment of T-cell-related diseases.
AE cells actively participate in inflammatory airway diseases by liberating mediators such as arachidonate metabolites and cytokines. Selective inhibition of PDE7 may be a useful anti-inflammatory approach for treating AE cells related diseases. B cells are well known key players in the allergic response, then selective PDE7 inhibitors are candidates for the treatment of B-cell-related diseases.
Thus, there is a need for selective PDE7 inhibitors, which are active at very low concentrations, i.e. preferably nanomolar inhibitors.
WO 88/01508 discloses compounds of formula
where R is H, alkyl, alkoxyalkyl, hydroxyalkyl, halo, cyano, carbamoyl, alkyl carbamoyl, formyl, alkylamino or amino;    X is —(CR4R5)a-NR6-(CR4R5)b-;    R1, R2, R3, and R5 are H or alkyl;    R4 and R6 are H, alkyl or aralkyl; a and b are 0, 1 or 2 and a+b=0, 1 or 2; R4 and R5 groups on vicinal carbon atoms may together form a carbon-carbon double bond; and geminal R4 and R5 groups may together form a spiro substitutent, —(CH2)d-, where d is 2 to 5; or a pharmaceutically acceptable salt thereof. These compounds are described as cardiotonics.
WO 00/66560 discloses compounds of formula

These compounds are described as progesterone receptor modulators.